Senescent cells look squashed in comparison to normal cells, most of which are round and smooth.

ABC: Catalyst

As well as not dividing, sleeping cells look very different to regular cells.

"They are very large and flat and have what we term a 'fried egg' morphology," Dr Chan said.

Some cells enter a sleeping state irreversibly — they will never divide again. However, some cells can wake up and revert to their original state.

What can happen to cells?

What can happen to cells?

Division: Cells in our body divide for growth and renewal. This usually happens through a process called mitosis.

Apoptosis: The regulated and "pre-planned" death of a cell. This is essential for normal growth and function and allows for the right amount of cells in the body. Different cell types have different "lifespans", meaning that after a certain number of days, they will undergo their pre-planned death. Apoptosis is orderly, and waste products from the dead cells are removed from the body.

Necrosis: Cell death caused by exposure, extreme conditions or toxins. Widespread necrosis can lead to serious disease.

Senescence: Cells that can no longer divide, but have not died. They remain in the body, but do not perform their previous functions.

A major reason our bodies evolved to have senescent cells is to suppress cancers, Dr Chan explained.

Genetic mutations triggered by things such as UV exposure can sometimes cause cells to replicate uncontrollably — and uncontrolled cell growth is cancer.

Cells are often able to detect these mutations and in responsego to sleepto stop them dividing.

The moles on our skin are hotspots for senescent cells for this reason. When the body senses genetic mutations caused by UV exposure, scientists believe cells then become senescent to shut down cancer growth.

Senescent cells also have a role in wound healing, embryonic development and recruiting immune cells to clear infection.

Moles are hotspots for senescent cells. By becoming senescent, sun-damaged cells are less likely to become cancerous.